Quick bursts of thoughts on Cespedes
My biggest concern would be the league getting used to him. I feel like that might explain why he did great in 2012 and 2015 with the Mets but only good in between. The AL figured out how to pitch to him. If that's the case then sometime next year the NL will too and the Nats will be left with a decent hitting OF (with a power focus), on a team full of decent hitters. That's not a bad thing, but Clint Robinson is a decent hitting OF. You aren't paying Cespedes 20 million a year or so just because he has an arm.
Of course Bryce plays CF in this case, right? I mean Cespedes is proven to have his issue. You aren't sitting Werth (not for Revere to play CF) so Bryce, more of an unknown gets it. I'm not as high on Bryce in center as some are but it's better than the other options.
Does someone get traded? MAT is the obvious choice because (1) he's the worst so Nats won't play him, and (2) he's the "prospect" so other teams will want him but I'm not sure I see that. Revere is only on a one-year deal, Werth is a perennial injury risk, and MATs defense is something the Nats need. If I were Rizzo I would want MAT around if only because he'll be a cheap 4th OF for a while. Werth of course is untradeable. 40 million for a DH who may or may not play and if he does play may or may not hit? No thank you. Revere? That's probably the best bet.
Biggest thing that's being undersold with Murphy and Revere (and maybe Cespedes)? Health. They should play which means you can more rely on their projections than Werth/Zimm/Rendon. This was important in losing Desmond - a very reliable guy playing time wise.
How does he effect the clubhouse? Well I'll guess he's probably not great in the clubhouse - he has been traded 3 times in 2 seasons and has surprisingly low FA interest. But it's the same in almost any situation. If the Nats win, which is more likely with him, they'll be fine with him (and fans will like him) if they don't win, that's when we see problems.
I don't mind 5 years covering the 30-34 seasons. That should be ok. The cost? Well we don't know 100 million would be too rich for me objectively. Subjectively of course - spend all your money.
We are mostly missing the second part of this as Donald noted - as a defensive move this matters too. Next year the competition for the NL East crown should be only the Mets, with an outside chance of the Marlins. If it's Nats or Mets then where Cespedes ends up counts double so to speak. Of course that should really only be figured as a set + for the team getting him in 2016 to be reviewed after every season.